Why is double spending a problem.
Suppose I have 1 bitcoin and I intend to send 1 btc to Alice and 1 btc to Bob.

Once I send 1 btc to Alice, UTXO of 1btc will be unlocked,transferred to Alice.No change is returned.Since Now when I try to send 1 btc to Bob, I have no UTXO to send it from. So the question of double spending is out of the question.

The double spending can be a problem when multiple transactions occur at the same time. If that is the case, why allow multiple transactions to occur at the same time?

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